Problem: Tiffany has taken 5 tests and her average score so far is 86. If she gets 100, a perfect score, on the remaining 2 tests, what will her new average be?
Solution: If she gets 100 on the remaining tests, the sum of her scores will be $5 \cdot 86 + 2 \cdot 100 = 630$ Her overall average will then be $630 \div 7 = 90$.